Notation, Network size – ZyXEL Communications ZyXEL Dimension ES-2108 User Manual
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ES-2108 Series User’s Guide
Appendix B IP Addresses and Subnetting
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Subnet masks can be referred to by the size of the network number part (the bits with a “1”
value). For example, an “8-bit mask” means that the first 8 bits of the mask are ones and the
remaining 24 bits are zeroes.
Subnet masks are expressed in dotted decimal notation just like IP addresses. The following
examples show the binary and decimal notation for 8-bit, 16-bit, 24-bit and 29-bit subnet
masks.
Network Size
The size of the network number determines the maximum number of possible hosts you can
have on your network. The larger the number of network number bits, the smaller the number
of remaining host ID bits.
An IP address with host IDs of all zeros is the IP address of the network (192.168.1.0 with a
24-bit subnet mask, for example). An IP address with host IDs of all ones is the broadcast
address for that network (192.168.1.255 with a 24-bit subnet mask, for example).
As these two IP addresses cannot be used for individual hosts, calculate the maximum number
of possible hosts in a network as follows:
Notation
Since the mask is always a continuous number of ones beginning from the left, followed by a
continuous number of zeros for the remainder of the 32 bit mask, you can simply specify the
number of ones instead of writing the value of each octet. This is usually specified by writing
a “/” followed by the number of bits in the mask after the address.
Table 85 Subnet Masks
BINARY
DECIMAL
1ST OCTET 2ND OCTET 3RD OCTET 4TH OCTET
8-bit mask
11111111
00000000
00000000
00000000
255.0.0.0
16-bit mask
11111111
11111111
00000000
00000000
255.255.0.0
24-bit mask
11111111
11111111
11111111
00000000
255.255.255.0
29-bit mask
11111111
11111111
11111111
11111000
255.255.255.248
Table 86 Maximum Host Numbers
SUBNET MASK
HOST ID SIZE
MAXIMUM NUMBER OF HOSTS
8 bits
255.0.0.0
24 bits
2
24
– 2
16777214
16 bits
255.255.0.0
16 bits
2
16
– 2
65534
24 bits
255.255.255.0
8 bits
2
8
– 2
254
29 bits
255.255.255.248 3 bits
2
3
– 2
6